Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 DOI 10.1186/s40623-017-0619-1

EXPRESS LETTER Open Access Anelastic properties beneath the – Tectonic Zone, Junichi Nakajima1* and Toru Matsuzawa2

Abstract We estimate the three-dimensional (3D) P-wave attenuation structure beneath the Niigata–Kobe Tectonic Zone (NKTZ), central Japan, using high-quality waveform data from a large number of stations. The obtained results confirm the segmentation of the NKTZ into three regions, as suggested by 3D seismic velocity models, and reveal character- istic structures related to surface deformation, shallow subduction of the Philippine Sea slab, and magmatism. The lower crust beneath the NKTZ west of the Itoigawa– Tectonic Line (ISTL) is overall characterized by distinct high attenuation, whereas the upper crust shows marked high attenuation to the east of the ISTL. Differences in the depths of anelastically weakened parts of the crust probably result in a first-order spatial variation in surface defor- mation, forming wide (width of ~100 km) and narrow (width of 25–40 km) deformation zones on the western and eastern sides of the ISTL, respectively. Many M 6.5 earthquakes occur in the upper crust where seismic attenuation in the underlying lower crust varies sharply, suggesting≥ that spatial variations in rates of anelastic deformation in the lower crust result in stress concentration in the overlying brittle crust. We interpret a moderate- to low-attenuation zone located in the lower crust at the northeast of Biwa Lake to reflect low-temperature conditions that are devel- oped locally as a result of shallow subduction of the cold Philippine Sea slab. Keywords: Seismic attenuation, Strain, Surface deformation, Slab, Grain size

Background crust facilitates the high strain rates along the NKTZ. The Niigata–Kobe Tectonic Zone (NKTZ) is a geo- However, geodetic observations with dense GNSS arrays detically derived high-strain-rate zone that extends for have revealed regional variations in the width of the ~500 km (NE–SW) across central Japan (Sagiya et al. NKTZ, ranging from tens kilometers to the east of the 2000) (purple shading in Fig. 1). The NKTZ is contract- Itoigawa–Shizuoka Tectonic Line (ISTL) to 50–100 km to ing in the WNW–ESE direction at a rate of ~107 year−1, the west of the ISTL (e.g., Sagiya et al. 2004; Ohzono et al. and the contraction rates are a few times higher than 2009; Nishimura et al. 2012). These observations suggest those observed in surrounding regions. The high strain that underlying physical mechanisms generating surface rates were first interpreted based on kinematic models, deformation are different between the west and east of such as a detachment model (Hirahara et al. 1998) and the ISTL. collision models (e.g., Shimazaki and Zhao 2000; Heki Nakajima and Hasegawa (2007) discussed the origin and 2001), but these models are unlikely to pro- and segmentation of the NKTZ on the basis of seismic duce the NKTZ due to the implausible physics required velocity structures and suggested that the crustal struc- to produce the combination of the observed stress field, ture west of the ISTL differs from that east of the ISTL. inferred mantle flow, and observed fault movement (Iio Jin and Aki (2005) reported a narrow, conspicuous low- et al. 2002). Iio et al. (2002, 2004) proposed an alternative coda attenuation zone west of the ISTL at frequencies model whereby deformation in the weak, ductile lower less than 4 Hz, whose existence was later corroborated by Carcole and Sato (2010) and Hiramatsu et al. (2013). *Correspondence: [email protected] Shear-wave splitting analysis of the upper crust sug- 1 Earth and Planetary Sciences, School of Science, Institute gests that the high strain rates at the surface west of the of Technology, Tokyo, Japan Full list of author information is available at the end of the article ISTL are caused by high deformation rates below the

© The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 2 of 9

Data and methods 133˚ 134˚ 135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 141˚ 39˚ Corner frequency M≥6.5 (1923-2015.2, Depth≤20 km) We used velocity waveform data from 8300 earthquakes (2 M 5) with focal depths shallower than 350 km 38˚ ≤ ≤ 100 km that occurred between October 2006 and February 2015 (Fig. 2a), and calculated spectral amplitudes of S-wave 37˚ NKTZ codas in 10 s time window taken at twice the theoretical S-wave travel time for a -dimensional seismic veloc- Lake Biwa ity model (Hasegawa et al. 1978). Spectral ratios were 36˚ ISTL Kanto calculated at common stations for earthquake pairs with a magnitude difference 0.5 and an inter-event distance ≥ 2 35˚ Tokai ≤40 km. We then fitted an ω source model (Brune 1970) Kinki to the averaged spectral ratio in the frequency range of 1–32 Hz and determined values of fc for the earth- 34˚ quake pair by a grid search with fc steps of 0.2 Hz. Fig- ure 3b shows an example of fc estimation procedure for 33˚ an earthquake pair. Values of fc were estimated to be 5.2 Fig. 1 Tectonic setting of central Japan. The Niigata–Kobe Tectonic and 15.4 Hz for M3.5 and M2.4 earthquakes, respectively. Zone (NKTZ) (Sagiya et al. 2000) is denoted by purple shading. Black Standard errors of fc evaluated by a bootstrap method triangles represent active volcanoes, and orange stars denote epicent- ers of earthquakes (M 6.5) that occurred at depths of 20 km with 1000 bootstrap samples were 0.27 Hz for the M3.5 ≥ ≤ during 1923–2015. Gray lines denote active faults. The Itoigawa–Shi- earthquake and 1.12 Hz for the M2.4 earthquake (Fig. 3c). zuoka Tectonic Line (ISTL) is shown by green curve. Names of regions We applied this procedure to available earthquake pairs discussed in the text are labeled and determined a pair of fc values. When five or more fc values were obtained from different pairs for an earth- brittle–ductile transition zone (Hiramatsu et al. 2010). quake, we calculated the average fc and used the value in These seismological observations obtained in the last the subsequent analysis. The total number of the earth- decade all support the model by Iio et al. (2002, 2004), quakes with fc estimates was 4718 (Fig. 2a). suggesting that concentrated deformation in the weak- Attenuation term and site amplification factor ened lower crust plays a crucial role in the generation of high strain rates west of the ISTL. For the earthquakes with fc estimates, we calculated Three-dimensional (3D) seismic velocity heterogenei- P-wave and noise spectral amplitudes from vertical com- ties have enhanced our understanding of spatial varia- ponent of velocity seismograms with a window length of tions in the elastic properties beneath high-strain-rate 2.56 s after and before the P-wave onset, respectively. We zones (e.g., Nakajima and Hasegawa 2007; Okada et al. limited our analysis to a frequency range with signal-to- 2014). However, the anelasticity associated with such noise ratios of ≥3. Frequency-dependent attenuation was zones is poorly understood. Anelasticity derived from assumed to obey the relationship ∗ ∗ −α intrinsic seismic attenuation is a key parameter for eval- t (t) = t0 f , uating ongoing viscoelastic deformation in the lower t∗ crust below high-strain-rate zones and mature fault sys- where 0 is the attenuation term at 1 Hz (Stachnik et al. tems (e.g., Eberhart-Phillips et al. 2014; Eberhart-Phillips 2004). We used α = 0.27 because α = 0.2–0.3 has been −1 estimated for the crust and the uppermost mantle 2016). Here we estimate the 3D P-wave attenuation (Qp ) structure in central Japan using waveform data recorded beneath the Japanese Islands (Nakajima et al. 2013; Naka- by a dense nationwide seismic network. We use a three- jima 2014; Saita et al. 2015) and this value range is the step approach developed by Nakajima et al. (2013) to most appropriate value for the mantle (e.g., Jackson et al. −1 2002). We constructed a set of observation equations for delineate the 3D Qp structure, which can minimize the potential trade-offs among unknown parameters. The multiple earthquakes at a single∗ station and carried out a simultaneous inversion for t0 and relative site amplifi- corner frequency (fc) of each earthquake is first estimated cation factors (for details, see Nakajima et al. 2013). As by the S-coda spectral ratio method. Next,∗ we perform a t values of fc estimated by the coda spectral ratio technique joint inversion for the attenuation term ∗ and site ampli- fication factors. Finally, the values oft are inverted for were for S waves, we calculated values of fc for P waves −1 by assuming the relationship f 1.33 f (Uchida the 3D Qp structure (e.g., Rietbrock 2001). The obtained cp = × cs attenuation model is discussed in the context of the et al. 2007). An example of spectral amplitudes observed deformation patterns observed within the NKTZ. at three stations for a single earthquake (EQ1) is shown Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 3 of 9

135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 38˚ a b

37˚

36˚

35˚

34˚ 050 100 150 200 Depth of hypocenters (km)

Fig. 2 a Distribution of hypocenters. Gray circles denote 8300 earthquakes used for estimates of corner frequencies, while colored circles represent 4718 out of 8300 earthquakes for which the corner frequency could be estimated. Circle colors correspond to earthquake focal depths. b Distribu- tion of seismograph stations (white squares) and horizontal grid nodes (blue crosses)

in Fig. 3d. Observed spectral amplitudes (blue curves in upper boundary of the Pacific slab (Nakajima et al. 2009)

Fig. 3d) are reproduced by theoretical spectral ampli∗ - were considered, adjacent horizontal grid nodes were tudes (red curve) that incorporated the estimated t0, site spaced at intervals of 0.2°, and vertical nodes were spaced amplification factors, and the ω2 source model. Relative at intervals of 5–50 km (Fig. 2b). We set initial values of −1 site amplifications estimated for the three stations are Qp to be 0.0033 for the crust and mantle wedge, and 0.001 shown in Fig. 3e. for the Pacific slab, and selected a damping parameter of

100 based on a trade-off curve∗ between the root mean Tomographic inversion t0 square (RMS) residuals of and model variance.∗ Final −1 t0 The 3D Q∗p structure was determined by the inversion of results were obtained after four iterations; RMS residu- 247,595 t0 values observed at 455 stations. It is noted that als decreased from 0.036 s in the initial model to 0.015 s. −1 Qp values estimated in this study correspond to those at We carried out two checkerboard resolution tests to 1 Hz. We calculated ray paths and travel times based on assess the reliability of tomographic results and con- the 3D P-wave velocity model of Nakajima and Hasegawa firmed that the obtained results are reliable beneath the (2007) with the ray tracing technique of Zhao et al. land area at a depth of <40 km (see Additional file 1). (1992a). In the inversion, we used a model parameteriza- We hereafter discuss the tomographic results in well- tion identical to that in Nakajima and Hasegawa (2007), resolved, unmasked areas in Fig. 4, where recovery rates where crustal discontinuities (Zhao et al. 1992b) and the are greater than 20%.

(See figure on next page.) t∗ Fig. 3 Examples of estimates of corner frequency (fc), attenuation term ( ), and site amplification factors. a Map showing the locations of two earthquakes (red stars) and three stations (blue squares). EQ1 is an earthquake with M3.5 and a focal depth of 62 km, and EQ2 is an earthquake with M2.4 and a focal depth of 46 km. b S-coda spectral amplitude ratios for a pair of EQ1 and EQ2. Gray and blue lines denote spectral ratios for various common stations and the averaged spectral ratio, respectively. The red line denotes the theoretical spectral ratio that best fits the observations. Val-

ues of fc estimated for the two earthquakes are indicated by green lines with fc values. Inter-event distances (D) and the number of common stations (N) are shown in the panel. c Uncertainties in fc estimates evaluated by a bootstrap method. Histograms of fc for (left) EQ1 and (right) EQ2 are plotted from 1000 bootstrap samples, and standard errors of fc are 0.27 Hz for EQ1 and 1.12 Hz for EQ2. d Spectral amplitudes of observed P waves (blue ∗ curves) and noise (dashed curves). Red line represents the theoretical spectral amplitude calculated from the optimal value of t0, site-amplification factors, and the ω2 source model. e Site amplification factors for the three stations Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 4 of 9

ab 135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 5.2Hz 15.4Hz 38˚ 103 EQ1 (H=62 km; M3.5)

EQ2 (H=46 km; M2.4) 37˚ 102 MATSUS DP.KTJ

36˚ 101 EQ1 N.AC2H Amplitude ratio 35˚ EQ2 100 D=31.8 km N=94 34˚ 10−1 110 Frequency (Hz) c 300 100 EQ1 (M3.5) EQ2 (M2.4)

200

50 # N 100

0 0 02 46810 10 12 14 16 18 20 Frequency (Hz) Frequency (Hz)

d MATSUS DP.KTJ N.AC2H 102 t*=0.065/Qave=488 t*=0.126/Qave=204 t*=0.049/Qave=426

100

−2

Spectral amplitud e 10

110 1101 10 Frequency (Hz) Frequency (Hz) Frequency (Hz)

e 102 n

100 Site ampficatio

10−2 110 1101 10 Frequency (Hz) Frequency (Hz) Frequency (Hz) Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 5 of 9

135˚ 136˚ 137˚ 138˚ 139˚ 140˚ 135˚ 136˚ 137˚ 138˚ 139˚ 140˚ a Depth = 5 km Depth = 10 km

37˚

36˚

35˚

34˚

38˚ Depth = 25 km Depth = 40 km B

37˚

36˚

35˚

Qp A 500 250 167 125 100

34˚ 0.002 0.006 0.010 P-wave attenuation

ISTL A B b 0 20 Moho 40 Philippine 60 Sea slab

Depth (km) 80

100 Region A Region BRegion C c 0 20

40

60

Depth (km) 80 −6 −3 036 Velocity perturbation (%) 100 0 100 200 300 400 Distance (km) Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 6 of 9

(See figure on previous page.) 1 Fig. 4 a Map showing Qp− at depths of 5, 10, 25, and 40 km. The color scale is shown in the right bottom panel. Black triangles represent active volca- noes. Gray dots at a depth of 10 km represent earthquakes (M 1) in a depth range of 5–15 km in 2003–2015, and stars at depths of 10 and 25 km ≥ denote earthquakes (M 6.5) at depths of 20 km from 1923 to 2015. Deep low-frequency earthquakes that occur within 2.5 km of each depth ≥ ≤ slice are indicated by white circles. The black dashed line shows the 40 km iso-depth contour of the upper surface of the Philippine Sea slab (Hirose et al. 2008; Nakajima et al. 2009). Areas with recovery rates of <20% for the two-grid-model of checkerboard resolution test are shaded in gray. b 1 Vertical cross section of Qp− along line A–B shown in the right bottom panel in a. Dashed lines indicate the Moho (Zhao et al. 1992b) and the upper surface of the Philippine Sea slab. The intersection of line A–B with the ISTL is indicated by a green arrow. Black triangles at the top denote active volcanoes. Gray dots and white circles represent earthquakes (M 1) and low-frequency earthquakes, respectively, that occurred within 5 km of line ≥ A–B. c Vertical cross section of P-wave velocity perturbations along line A–B (Nakajima and Hasegawa 2007). Other symbols are the same as in b

Tomography results and vice versa (Fig. 4b, c). The correlation coefficient −1 −1 The results show marked variations in Qp with depth between Qp and dVp at a depth of 25 km is calculated −7 (Fig. 4a). At a depth of 5 km, high-attenuation areas are to be −0.47 with a probability value of ~10 , indicating distributed mainly in Kinki and Tokai districts, and a moderately weak but statistically significant negative around active volcanoes. In contrast, areas of moderate to relationship. low attenuation are dominant at a depth of 10 km, even To the east of the ISTL (region C), the upper crust shows though isolated high-attenuation areas are observed in high attenuation and low velocity, while the lower crust Kanto, Tokai, and Kinki districts. Seismicity in the upper shows low attenuation and moderate to high velocity (see crust and earthquakes with M ≥ 6.5 (white stars in Fig. 4) dark symbols in Fig. 5), which differentiates seismic char- tend to occur in areas of low to moderate attenuation. The acteristics of the lower crust west of the ISTL. The high- lower crust, at a depth of 25 km, shows high attenuation attenuation, low-velocity upper crust probably reflects a in Kinki, southeast of Biwa Lake, and around volcanic thick (~6 km) accumulated basin fill (e.g., Sato 1994). Seis- areas in the northern part of the study area. On the whole, mic anisotropy derived from shear-wave splitting analysis the NKTZ shows high attenuation west of the ISTL, but reveals that the anisotropy east of the ISTL is structurally there is a moderate- to low-attenuation area northeast of induced, which can be explained by intense faulting and Biwa Lake, surrounded by high-attenuation areas. folding in the sedimentary basin (Hiramatsu et al. 2010). −1 Figure 4b shows a vertical cross section of Qp along The volcanic area (region B) shows high attenuation and the NKTZ (line A–B shown in the panel for 40 km low velocity throughout the crust. High attenuation in this depth), and characterizes attenuation features in the crust area is also suggested from S-wave attenuation studies of the NKTZ. Marked high-attenuation areas exist in the (e.g., Carcole and Sato 2010; Liu and Zhao 2015) and low- lower crust at horizontal distances of 0–120 km and 200– frequency (1–4 Hz) coda-wave attenuation (e.g., Jin and 300 km, while the lower crust shows low attenuation east Aki 2005; Hiramatsu et al. 2013). As Vp/Vs values in the of the ISTL at distances of >370 km. In volcanic areas lower crust are high (~1.85; Nakajima and Hasegawa 2007) located at horizontal distances of 300–370 km, high- and a low-resistivity body has been reported at depths of attenuation areas extend continuously from the lower 20–30 km (e.g., Ogawa and Honkura 2004), we infer that crust to the surface. The resolution of the uppermost the existence of magmatic fluids is responsible for the mantle is not sufficient to discuss the observed attenu- high-attenuation and low-velocity anomalies. The mag- ation structure, as a result of an insufficient number of matic fluids probably originate from dehydration-related rays propagating at depths >60 km, where the recovery fluids in both the Philippine Sea and Pacific slabs (Naka- of the checkerboard resolution tests is not good (Fig. 4b; mura et al. 2008). The possibility of the upward migra- Additional file 1: Figure S1). tion of slab-derived fluids is supported by high 3He/4He ratios (~8 Ra) (Umeda et al. 2013). The absence of appar- Discussion ent high-attenuation areas beneath the Moho is probably One of the most striking features revealed by this study is due to the insufficient number of rays propagating through the segmentation of the NKTZ derived from the attenu- the uppermost mantle, as a consequence of small number ation structure in the crust. Nakajima and Hasegawa of deep-focus earthquakes with reliable fc estimates; this (2007) proposed that the NKTZ can be divided into a inference is supported by the poor recovery rates of the non-volcanic area (region A), a volcanic area (region B), checkerboard patterns at depths of >60 km (Fig. 4b). and an area east of the ISTL (region C) (Fig. 4c). The dis- On the whole, the lower crust beneath region A shows −1 tribution of Qp along the NKTZ correlates well with that a marked low-velocity and high-attenuation anomaly of P-wave velocity perturbations (dVp) in the sense that (Fig. 4b, c). Since heat flow measurements do not sug- high-attenuation areas correspond to low-velocity areas gest high-temperature conditions within the NKTZ Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 7 of 9

ab 12 CC = –0.47 East of the ISTL

6

0

−6 P-wave velocity perturbation (%) −12 Northeast of Biwa Lake

0.000 0.004 0.008 0.012 0.000 0.004 0.008 0.012 P-wave attenuation P-wave attenuation

0 80 160240 320400 480 Distance from point A (km) Fig. 5 a Relationship between P-wave attenuation values obtained in this study and P-wave velocity perturbations (Nakajima and Hasegawa 2007) at a depth of 25 km for each grid node located beneath the NKTZ (purple shading in Fig. 1). The number of grid nodes shown is 109. The cross- correlation coefficient for the two data sets is 0.47. Colors represent the distance along the NKTZ calculated from point A in line A–B (Fig. 4). b − Relationship between P-wave attenuation values obtained in this study and P-wave velocity perturbations that are averaged at every 20 km along line A–B. Bars denote 1-sigma uncertainties

(Tanaka et al. 2004), the presence of aqueous fluids prob- regional tectonic stresses promote anelastic deforma- ably results in reduced seismic velocities and enhanced tion in the weakened lower crust and that strain rates seismic attenuation (Toksöz et al. 1979). However, a enhanced by the deformation reduce grain size of min- low-attenuation area exists at horizontal distances of erals (Platt and Behr 2011). The highly attenuated lower 130–180 km, where the Philippine Sea slab is in contact crust observed beneath the NKTZ west of the ISTL is with the continental Moho (Fig. 4b). It is known that a probably affected by grain size reduction due to localized, temperature decrease by ~100 °C reduces seismic veloc- high-strain-rate deformation, because grain size reduc- ity by only ~1% (e.g., Duffy and Anderson 1989), but tion enhances seismic attenuation (Jackson et al. 2002). attenuation in olivine could be halved under upper man- One important implication of our attenuation model tle conditions (e.g., Jackson et al. 2002). Although few is that surface deformation northeast of Biwa Lake, systematic experiments have investigated the tempera- where the lower crust shows moderate to low attenua- ture dependence of seismic attenuation in lower crustal tion, would differ from that of other parts of the NKTZ, materials, we infer that the lower crust locally cooled by because marked anelastic deformation of the lower crust shallow-angle subduction of the Philippine Sea slab is the is unlikely to occur in this region. primary cause of the low to moderate seismic attenua- Spatial variations in the weakness of the lower crust tion values observed in this part of the crust. The inferred would cause spatial gradients in anelastic deformation low-temperature conditions in the crust northeast of rates, and stress could be consequently concentrated Biwa Lake are supported by deep cutoff depths of crustal in the overlying brittle layer. The occurrence of many earthquakes (Omuralieva et al. 2012). M ≥ 6.5 earthquakes in the upper crust, below which The depth variations in the anelastic structures can anelastic properties in the lower crust vary sharply qualitatively account for regional variations in the (Fig. 4a), may be one of the manifestations of high stress width of the high-strain-rate zone. A narrow (25–40 km concentration in the brittle upper crust. While it is con- wide) zone of surface deformation on the Echigo plain sidered that reduction in the effective normal stress on (Nishimura et al. 2012), east of the ISTL, is probably due a fault as a result of the supply of overpressurized fluids to the shallow deformation of the thick, anelastically plays key roles in facilitating crustal earthquakes (e.g., weakened sediment. In contrast, the NKTZ west of the Sibson 1992; Hasegawa et al. 2005; Yoshida et al. 2014), ISTL, which has a width of ~100 km, is caused by high our observations add growing body of evidence that ane- rates of anelastic deformation in the lower crust, as pro- lastic deformation in the lower crust is equally important posed by Iio et al. (2002, 2004). It is likely that long-term in seismogenesis in the upper crust. Nakajima and Matsuzawa Earth, Planets and Space (2017) 69:33 Page 8 of 9

Conclusions basis of the multiple lapse time window analysis of Hi-net data. Geophys J Int 180:268–290 This study characterizes seismic attenuation structures in Duffy TS, Anderson DL (1989) Seismic velocities in mantle minerals and the central Japan and discusses spatial relationship between mineralogy of the upper mantle. J Geophys Res 94:1895–1912 Eberhart-Phillips D (2016) Northern California seismic attenuation: 3D Qp an attenuation structures and surface deformation along Qs models. Bull Seismol Soc Am 106:2558–2573 the NKTZ. The anelastically weakened lower crust west Eberhart-Phillips D, Bannister S, Ellis S (2014) Imaging P and S attenuation in of the ISTL promotes surface contraction over a region the termination region of the Hikurangi subduction zone, New Zealand. Geophys J Int 198:516–536 ~100 km wide, while anelastic deformation in the shallow, Hasegawa A, Umino N, Takagi A (1978) Double-planed structure of the deep thick sedimentary layer west of the ISTL causes a nar- seismic zone in the northeastern Japan arc. Tectonophysics 47:43–58 row region of surface deformation. Our observations can Hasegawa A, Nakajima J, Umino N, Miura S (2005) Deep structure of the north- eastern Japan arc and its implications for crustal deformation and shallow explain qualitatively the regional variations in the width seismic activity. Tectonophysics 403:59–75 of the high-strain-rate zone across the ISTL. The attenua- Heki K, Miyazaki S (2001) Plate convergence and long-term crustal deforma- tion structure revealed in this study provides practical and tion. Geophys Res Lett 28:2313–2316 Hirahara K, Ando M, Hoso H, Wada Y, Nakano T (1998) Search for the move- important constraints on quantifying subsurface defor- ment of an active fault by GPS measurements. Earth Mon 225:149–153 mation, which is essential to constructing realistic models (in Japanese) of seismogenesis in high-strain-rate zones. Hiramatsu Y, Iwatsuki K, Ueyama S, Iidaka T, the Japanese University Group of the Joint Seismic Observations at NKTZ (2010) Spatial variation Additional file in shear wave splitting of the upper crust in the zone of inland high strain rate, central Japan. Earth Planets Space 62:675–684. doi:10.5047/ eps.2010.08.003 Additional file 1. Figure S1. Result of cherckerboard resolution tests for Hiramatsu Y et al (2013) Spatial variation in coda Q and stressing rate around a the one-grid model and b the two-grid model. See text for details. the Atotsugawa fault zone in a high strain rate zone, central Japan. Earth Planets Space 65:115–119. doi:10.5047/eps.2012.08.012 Hirose F, Nakajima J, Hasegawa A (2008) Three-dimensional seismic velocity structure and configuration of the Philippine Sea slab in southwestern Abbreviations Japan estimated by double-difference tomography. J Geophys Res NKTZ: Niigata–Kobe Tectonic Zone; ISTL: Itoigawa–Shizuoka Tectonic Line. 113:B09315. doi:10.1029/2007JB005274 Iio Y, Sagiya T, Kobayashi Y, Shiozaki I (2002) Water-weakened lower crust and Authors’ contributions its role in the concentrated deformation in the Japanese Islands. Earth JN performed data processing and tomographic inversions. Both JN and TM Planet Sci Lett 203:245–253 designed this study and contributed to the interpretations and preparation of Iio Y, Takeshi S, Kobayashi Y (2004) Origin of the concentrated deforma- the manuscript. Both authors read and approved the final manuscript. tion zone in the Japanese Islands and stress accumulation process of intraplate earthquakes. Earth Planets Space 56:831–842. doi:10.1186/ Author details BF03353090 1 Earth and Planetary Sciences, School of Science, Tokyo Institute of Technol- Jackson I, Fitz Gerald JD, Faul UH, Tan BH (2002) Grain-size sensitive seismic ogy, Tokyo, Japan. 2 Research Center for Prediction of Earthquakes and Vol- wave attenuation in polycrystalline olivine. J Geophys Res 107(B12):2360. canic Eruptions, Graduate School of Science, Tohoku University, , Japan. doi:10.1029/2001JB001225 Jin A, Aki K (2005) High-resolution maps of coda Q in Japan and their interpre- Acknowledgements tation by the brittle–ductile interaction hypothesis. Earth Planets Space We used waveform data recorded at a nationwide seismograph network. All 57:403–409. doi:10.1186/BF03351825 the figures in this paper were plotted using GMT (Wessel and Smith 1998). Liu X, Zhao D (2015) Seismic attenuation tomography of the southwest Japan Constructive reviews by T. Yamada and an anonymous reviewer improved arc: new insight into subduction dynamics. Geophys J Int 201:135–156 the manuscript. This work was supported by JSPS KAKENHI Grant Numbers Nakajima J (2014) Seismic attenuation beneath Kanto, Japan: evidence for JP16H04040, JP16H04071, JP26109002 and JP26109006, and by the Ministry high attenuation in the serpentinized subducting mantle. Earth Planets of Education, Culture, Sports, Science, and Technology of Japan, under its Space 66:12. doi:10.1186/1800-5981-66-12 Observation and Research Program for the Prediction of Earthquakes and Nakajima J, Hasegawa A (2007) Deep crustal structure along the Niigata–Kobe Volcanic Eruptions. Tectonic Zone, Japan: its origin and segmentation. Earth Planets Space 59:e5–e8. doi:10.1186/BF03352677 Competing interests Nakajima J, Hirose F, Hasegawa A (2009) Seismotectonics beneath the Tokyo The authors declare that they have no competing interests. metropolitan area: effect of slab–slab contact and overlap on seismicity. J Geophys Res 114:B08309. doi:10.1029/2008JB006101 Data availability Nakajima J, Hada S, Hayami E, Uchida N, Hasegawa A, Yoshioka S, Matsuzawa The data that support the findings of this study are available from the cor- T, Umino N (2013) Seismic attenuation beneath northeastern Japan: responding author on request. constraints on mantle dynamics and arc magmatism. J Geophys Res Solid Earth. doi:10.1002/2013JB010388 Received: 11 January 2017 Accepted: 13 February 2017 Nakamura H, Iwamori H, Kimura J-I (2008) Geochemical evidence for enhanced fluid flux due to overlapping subducting plates. 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